1. Field of the Invention
This invention relates to a method for agglomerating powdered carboniferous materials by compaction and, more particularly, relates to a method for agglomerating powdered coal. This invention further relates to a composition of matter consisting of the agglomerated material resulting from application of the method.
2. Description of the Prior Art
Mined coal is used principally as a source of heat energy in power plants. Most of the coal which is shipped to power plants consists of lump or nut coal (i.e. particles which will not pass through a 3/4" screen). Power plants pulverize such lump or nut coal upon its receipt and blow the powdered coal into their furnaces. Powdered coal produced by the process of mining or purifying the coal is also shipped to power plants for consumption in their furnaces.
For many years it has been known that powdered coal (i.e. coal dust) is produced in great quantities during the mining of coal, during its handling before and after shipment, and during shipment and storage. This powder is the result of the purely mechanical effects of mining and transporting the coal. It is due as well to weathering and oxidation of stored coal.
Large quantities of powdered coal may also be created by mechanical processes for removing sulfur products from coal. One of the major sources of sulfur in coal is pyrite, which frequently occurs as tiny particles dispersed throughout the coal matrix. Hence, to be cleaned to acceptable levels of sulfur and ash, coal from some mines must be finely ground to liberate pyrite and ash minerals from the coal particles. This operation is followed by separation of the contaminants from the coal by physical (e.g. froth flotation) or chemical processes, which leaves a cleaned coal product in powdered form.
The formation of powdered coal in any of these various ways leads to several significant problems. Coal which is reduced to powder is difficult to ship or use. When powdered coal cannot be shipped or used, its economic value and energy value is totally lost and grinding processes for removing sulfur are useless, unless performed at a power plant site.
Powdered coal (i.e. coal dust) is highly flammable and explosive. Powdered coal may be accidentally ignited while being transported. For example, it may be ignited while being transported on conveyor belts from mines to coal storage warehouses which are suspended above railroad tracks, e.g., by sparks from a railroad car's bad roller bearings.
Powdered coal can also lead to environmental disturbances, including both threats to health and vegetation and aesthetic damage. Powdered coal can permeate the air of mining sites or other work sites, and along transportation routes, should it seep out or blow out of transport vehicles. It can settle on vegetation, in dwellings or anywhere. The problems of black lung are well known; the aesthetic problems are apparent. As coal is shipped greater and greater distances from newly opened mines to old population centers, the exposure of the environment is increased.
Powdered coal is not ordinarily stored; however, it tends to accumulate within various areas of a mine's shipping structures. These structures must be continually washed down and the powdered coal removed before spontaneous combustion occurs. Because of its large surface area, powdered coal reacts with oxygen to produce great amounts of heat. The relative compactness of piles of powder prevents convection currents of air from carrying away heat generated in a pile. Yet, there is generally enough oxygen in ambient air trapped between particles of powder to support spontaneous combustion. As a result, there is a danger of spontaneous combustion when powdered coal accumulates.
Many shipping mines collect coal dust in "bag houses" especially equipped with fans to pull coal dust from the air through collection bags which trap the coal dust. The bags are then shaken down and the coal dust is placed on conveyers to be loaded into railroad cars for shipping. Because of the danger of spontaneous combustion, as little coal dust as possible is stored or stockpiled over long periods of time.
An additional problem is caused merely by the accumulation of coal dust. When coal dust is produced and cannot be collected and used, disposal becomes a problem. Equipment or entire areas may become clogged with the powder. As with any solid waste, disposal must be done in a safe and inexpensive manner.
While the above problems are most familiar in connection with coal, they may also arise in connection with lignite or shale or other carboniferous materials.
At present, certain means and methods exist for dealing with at least some of the above-mentioned problems of powdered coal and other carboniferous materials. Accumulations of powdered coal in storage areas or in shipping areas may be sprayed with water to reduce oxidation and dispersal. Vacuum or air filter systems may be used to collect the powder and reduce its dispersal. Such measures, however, may be ineffective to prevent spontaneous combustion, may do little or nothing about the problem of dispersal, and may be expensive, as illustrated by the use of "bag houses." Such measures also leave the coal in dust form.
It is known to produce blocks or other agglomerated shapes made from particles of certain combustible materials. For example, a pelletized fuel sold under the name Woodex is made from organic vegetable material rich in cellulose, such as wood chips, sawdust, bagasse (sugar cane residue) and corn husks. A process is used on these materials combining high pressure, relatively high temperature and the presence of moisture. Similarly, artificial logs are produced by compressing under high pressure wood waste, such as logged wood, wood chips or coarse sawdust. Coal is a constituent of one such product, made in brick form from hardwood chips and coal chunks held together by an organic resin. Other products, sold under the names Coalex or Coallog, also exist, which are made of coal and a binder consisting of pulverized biomass such as wood chips or other organic waste.
One known process, disclosed in U.S. Pat. No. 4,230,460, issued Oct. 28, 1980, provides for converting powdered coal into pellets by utilizing lime (or limestone or dolomite) and flyash as binders and compressing the resultant mixture. The pellets formed by this process, however, are principally useful as a fuel in fluidized bed combusters. The lime functions as an in-furnace absorbent for sulfur dioxide. The flyash is an inert ingredient which is captured downstream as an undesirable particulate pollutant by electrostatic precipitators or other recovery devices.
A study of the prior art shows that a primary problem in dealing with powdered coal is how to form it into usable larger bodies. Apparatus for agglomerating non-carboniferous materials by compaction are well known and discussed in C. Holley, BRIQUETTING AND COMPACTING (distributed by Ferro-Tech, 467 Eureka Road, Wyandotte, Mich. 48192) and C. Holley, AGGLOMERATION-THE STATE OF THE ART (distributed by Ferro-Tech, supra), both articles incorporated herein by reference. Another article, C. Holley & J. Antonetti, AGGLOMERATION OF COAL FINES (distributed by Ferro-Tech, supra, and presented at the 15th Biennial Conference of IBA, Montreal, Quebec 1977), also incorporated herein by reference, discusses the use of known apparatus and the application known technology to the difficult task of agglomerating fines whose size are less than 28 mesh, e.g. powdered coal.
The latter article discloses that certain extruder apparatus have been utilized for agglomerating by compaction powdered coal having an ambient moisture content up to 30%. However, even with such a high moisture content, a binder has been used, because extruder apparatus generally do not exert enough pressure to force the coal particles to compact and bond together. This indicates that ambient moisture alone does not function well as a binder in the pressure range of such apparatus.
Not only does conventional agglomeration of coal dust by extruder apparatus require a binder, such apparatus have also been of limited use because (1) they are ineffective for handling powdered coal having a low ambient moisture content and (2) the primary sources of powdered coal are bag houses which yield filter cakes having low ambient moisture contents. Extruder apparatus form agglomerated solids, such as pellets, by delivering the powdered coal to a die through which it is compressed. Powdered coal having a low moisture content is difficult to convey and control. It easily becomes airborne dust and can lead to explosion danger, especially around such compacting apparatus. Also, powdered coal having a low moisture content will not flow readily through a die but rather tends to pack and degrade the die. Finally, powdered coal having a low moisture content, such as filter cake coal taken from a bag house, is not easily moistened because water does not mix well with relatively dry powdered coal.
The Holley and Antonetti article further indicates that the greatest success in agglomerating powdered coal has been achieved in several "dry" processes. A briquette compactor and a disc pelletizer (the latter apparatus being an agitator rather than a compactor) have been used, employing binders such as lignosulfonate, bentonite, tar, pitch, and fuel oil. The principal disadvantages to the use of these two types of apparatus lie in the cost of the apparatus and their operation, as well as the cost of the binders used in the process. Moreover, the dry powdered coal presents a significant handling problem here also, in that it easily becomes airborne dust and can explode.
In summary, conventional coal agglomeration processes utilize pulverized or lump coal, and none of them produce pellets or bricks principally from powdered coal. The known processes involve use of significant quantities of binders, and some processes require complex agglomeration steps involving heat and pressure. To the extent such processes could be used as a partial solution to the problems caused by powdered coal, they appear to involve significant expense. Moreover, some of the processes produce products of limited use. The pellets produced according to the process disclosed in U.S. Pat. No. 4,230,460 are principally useful in fluidized bed combusters and result in a significant increase in flyash pollution. Other products are designed primarily for home fireplace use.